Electric detonator



Dec. 31, 1940. 1 B. wooDwoRTH 2 2,226,988

ELECTRIC DETONATOR Filed Aug. 9, 1939 Patented Dec. 31, 1940 UNITED STATES 2,226,9ss l ELECTRIC Du'rowrronIV Leon Byron Woodworth, Durban, Natal, Union of South Africa, assignor of an undivided share to Central Mining and Investment Corporation,

Limited, Johannesburg, Union of South Africa, a limited-liability company of Great Britain Application August 9, 1939, Serial No. 289,315 In the Union of South Africa August 19, 1938 1 Claim;

V'I'his invention relates to electric detonators and generally to electrically energized means,

- -for bringing about the detonation of an explosive charge.

It is believed that full realization of the benefits derivable from electrical blasting is hampered by the fact that the electrical detonators commonly used heretofore are susceptible to untimely firing by stray electrical currents and particularly by sparking resulting from static charges which are apt to beset up by electrical storms.

Electrical blasting usually comprises a train of operations beginning with the heating of a resistance by the blasting current. The heated resistance causes'defiagration of a deagrating element which is usually formed as a pill seated on the resistance. In instantaneous detonators said deflagration acts directly on the detonating compound 'to cause it to detonate. On the other hand in delay action detonators the deagration ignites a combustible delay member which takes time to burn through and then spits onto the detonating charge to cause its detonation, thus enabling the dierent shots of a group to be iired simultaneously but to explode at different times.'

It will be seen that thistrain of operations comprises a rst step of electrical heating and a series of subsequent steps which may conveniently be referred to as ame steps and which are out of control in the sense that the commencement of the rst of them inevitably leads to the culmination of the last of them. The dangerous feature of the train is that electrical phenomena other than the passage of the regular blasting current through the resistance, for instance sparking as above described, can take the place of the regular electrical heating by the blasting current.

.An object of the invention is to provide an electric detonator in which a resistance heated by the blasting current in turn heats an electrically inert or non-conducting body and thereby renders it a conductor, this operation occupying a substantial period of time; whereupon said body admits the heating current through itself and, being thereby further heated, performs the first of the ame steps.

Two constructional forms of the invention are illustrated in the drawing in which Figure 1 is a longitudinal sectional view of a detonator constructed according to the invention, and

Fig. 2 is a longitudinal sectional view of a detonator similar to Fig. 1 1 but incorporating a safety fuse between the resistance to be heated by the blasting current and the detonating mass.

In the drawing 2 indicates the detonator casing. The end 3 of the casing may be closed as shown in Fig. 1 and contain the vdetonating mass 4, with or without a combustible delay element 5; or said end may be open as shown in Fig. 2 and have fitted into it the end of a safety fuse 6 leading to another casing 1 which contains the detonating mass I, with or without a combustible delay element 5. 10

The other end 8 is provided with a closing member, such as the cap 9 or a plug, which is sealed with respect to the casing after the assembly is made. Electrical leads I0 of ample section enter the casing through said member 15 and are connected to the ends of a heating resistance II, which may be a tungsten coil. Said resistance is embedded in a cylindrical or other shaped mass I2 of material which provides the rst of the ame steps and which also causes the 9 delay.

Material suitable for the purpose includes the oxides of thorium and cerium and other metals of the same class. These have the characteristics of being electrically non-conductive when 25 cold, but serve as conductors when heated. In such case the bare conductors leading to the coil are embedded in the cylinder material at the ends thereof so that the cylinder and the resistance are electrically in parallel with one 3" another. The result is that when the blasting current is passed through the leads, the rst effect is the heating of the coil II which in turn heats the cylinder I2. The latter thereupon becomes a conductor and the mass I2 is heated to 35 incandescence by the current passing there.I through. The cylinder is in contact with or suiiiciently near the means of producing the next flame step, such as the detonating mass 4 or the delay compound 5 of Fig. 1 or the safety 40 fuse 6 of Fig. 2 so as to cause the said means to burn or detonate as the case may be, and thus directly or indirectly but positively bring about the detonation of the detonating compound. If

desired, combustible material may be incorporated in the rare earth material in order to ensure the transmission of the flame operation to the next flame step; provided such material is electrically non-conductive when cold or present in nsumcient quantity to make the cylinder conductive when cold.

The delay period is that necessary for the coil II to heat the rare earth cylinder I2 to the state of conductivity and for the current which then flows through the cylinder material to heat the latter to incandescence or at least to a temperature suilicient to cause ignition of the aforesaid admixed combustible material, when such is present. If a current able to heat the resistance il endures for a lesser period than such delay period, the name operation does not commence and detonation does not occur.

y In another instance the cylinder I2 is made of magnesite having acombustible substance, for instance magnesium metal powder, incorporated in lts external surface layer. In that case since the magnesite does not become electrically conductive upon beingr heated, but is a poor conductor of heat, the delay is due to the time period necessary for the coil to heat the magnesite sumciently to bring about combustion of the combustible admixture.

In making use of the invention, the means for supplying the ring current are appropriately arranged so as not to be subjected to undue electrical stress by reason of the prolonged period during which the ring current is maintained. Primary or secondary electrical batteries may for instance be employed as the current source.

If the current source is a main supply, the transformer may be provided with suitable reactance such as 14% or more; or a motor-generator set' may be employed. r

I claim:

An electrical detonator comprising an electrical circuit including a resistance to be heated by passage of electric current through the circuit, a detonating mass in a non-operative relationship to said resistance, a member positioned to be pre-heated by said resistance and comprising a substance the electrical resistance of which.decreases when the member is so preheated, means for passing electric current through the pre-heated member and thus heating it further until it becomes activated, said member being arranged to cause detonation of said mass when activated, the arrangement being such that the activation of said member occupies a period of time substantially longer than the probable duration of any adventitious electrical condition which is likely to occur and which tends to activate said member IEON BYRON WOODWORTH. 

